Chromium-chromium carbide powder and article made therefrom

ABSTRACT

1. A POWDER CONTAINING FROM ABOUT 0.2 WT. PERCENT TO ABOUT 5.4 WT. PERCENT CARBON AND WHEREIN SUBSTANTIALLY EVERY PARTICLE OF SAID POWDER CONSISTS ESSENTIALLY OF CHROMIUM AND AT LEAST ONE CHROMIUM CARBIDE TAKEN FROM THE CLASS CONSISTING OF CR23C6;CR7C3 AND CR3C2.

Nov. 5, 1974 J. F. PELTON o canomuwcnaomum CARBIDE POWDER AND ARTICLEMADE 'rnmnmmom Filed Aug. 15, 1973 :5 sheets-sheet 1 CARBIDES CHROMIUMCARBIDES CARBIDES CHROMIUM CHROMIUM Nov. 5, 1974 J. F. PELTONCHROMIUMCI'IROMIUM CARBIDE POWDER AND ARTICLE MADE THEREFROM Filed Aug.15, 1975 3 Sheets-Sheet 2 2 Weight C in Powder wo p aum o 4003 109 Nov.5, 1974 J. F. PELTON 3,346,034

CHROMIUM-CHROMIUM CARBIDE POWDER AND ARTICLE mum mnnnmou Filed Aug. 15,1973 3 Sheets-Sheet 5 8 o o o 8 o Q (I) (0 1 United States Patent3,846,084 CHROMIUM-CHROMIUM CARBIDE POWDER AND ARTICLE MADE THEREFROMJohn Franklin Pelton, Yorktown Heights, N.Y., assignor to Union CarbideCorporation, New York, N.Y. Filed Aug. 15, 1973, Ser. No. 388,433 Int.Cl. B231) 3/00 US. Cl. 29-19L2 12 Claims ABSTRACT OF THE DISCLOSURE Acomposite powder for use in producing articles or coatings having uniquewear and frictional characteristics consisting essentially of a chromiummatrix with at least one chromium carbide taken from the class ofcarbides consisting of Cr C Cr C and Cr C and each particle containingfrom about 0.2 wt. percent to about 5.4 wt. percent carbon.

Also described are processes for making the powder and an article coatedby passing the powder through a plasma or detonation gun.

This invention relates to a novel powder for use in producing articlesand coatings having unique wear and frictional characteristics. Moreparticularly this invention relates to powders which are to be appliedas a coating on a substrate using metal spraying techniques and to thearticles and coatings made thereby.

Chromium metal has been used as an electroplated coating (i.e., hardchromium plating") for many years to restore Worn or damaged parts totheir original dimensions, to increase wear resistance, reduce friction,and provide corrosion resistance. Chromiums excellent wear andfrictional characteristics have been attributed to its low ratio ofenergy of adhesion to hardness when mated against a number of materialsthat are commonly used in engineering applications. Hard chromiumelectroplate, however, has a number of limitations. The electroplatingof chromium is economically feasible when the configuration of the partis relatively simple and the number of the parts and/ or their size isrelatively small. When the configuration of the part becomes complex.obtaining a uniform coating thickness by electro-deposition is ditficultand requires precise placement of electrodes and thieves. Without auniform coating thickness, grinding to a finished surface configurationbecomes necessary, and it is both difficult and expensive withelectroplated chromium because of its inherent brittleness and hardness.The rate of deposition by electroplating is relatively low, and thus fora large number of parts and/or large areas and/or thick coatings a verysubstantial capital investment in plating tanks and power supplies isrequired. In chromium electroplating it is often necessary to useexpensive surface cleaning and etching procedures to prepare substrates.Further, with many substrate materials it is not possible to directlyapply chromium electroplating and one or more undercoats of other metalsmust be used. Spent plating baths present a disposal problem becausethey are a serious pollution source, and hence handling them addssignificantly to the cost of the process.

An alternative method of depositing chromium metal is by metal sprayingsuch as with a plasma or detonation gun. These methods offer a number ofprocessing advantages. Surface preparation is relatively simple andinexpensive. The coatings can be applied to almost any metallicsubstrate without using undercoats. The rate of deposition is very highso that a large volume of parts can be coated with a minimal capitalinvestment. The coating thickness can be controlled very closely so thatany subsequent finishing can be kept to a minimum. The

overspray can be easily contained and recovered making pollution controla simple matter.

Unfortunately, plasma-deposited chromium is not as wear-resistant atambient temperature as hard electroplated chromium. This is because thewear-resistance of chromium plate is not an inherent property ofelemental chromium but is believed to arise largely from impurities andstresses incorporated in the coating during plating. Plasma depositedchromium being a purer form of chromium thus lacks the wear resistancesof hard chromium plate while retaining the corrosion-resistancecharacteristics of chromium.

It has now been discovered that coatings made by the plasma ordetonation-gun process can be made that are remarkably superior to hardchromium electroplate in compatibility, frictional characteristics andwear resistance by incorporating a dispersion of chromium carbideparticles in a chromium matrix.

Coatings of this type have been made from mechanical mixtures of powdersas described in my co-pending ap plication Ser. No. 388,434 filed Aug.15, 1973. While such mechanical mixtures are advantageous, there arecertain limitations to the quality of coatings made from them. Bothplasma and detonation-gun deposition result in a coating with amultilayer structure of overlapping, thin, lenticular particles orsplats. Each coating particle or splat is derived from a single particleof the powder used to produce the coating. There is little, if any,combining or alloying of two or more powder particles during the coatingdeposition process. This results in some of the splats being completelychromium and some being completely chromium carbide, with the finenessor interparticle spacing being controlled by the sizes of the initialchromium and chromium carbide powder particles. Thus, the fineness ofthe chromium carbide dispersion in the coating is limited by thefineness of the powder that can be handled by the coating process. Sincemany desirable properties of the coating are improved by reducing theinterparticle spacing or increasing the fineness of the dispersion andsince it is desirable from a coating application standpoint to usepowders with particles much larger than desired from the coatingfineness standpoint, it would be advantageous to produce a coating inwhich each splat is a mixture of chromium and chromium carbide. This inturn requires that each powder particle contain a mixture of chromiummetal and chromium carbide.

Accordingly, it is an object of this invention to provide a powderwhich, when sprayed by a plasma or detonation-gun, will produce anarticle or coating wherein each splat is a mixture of chromium metal andchromium carbides.

Another object is to provide such a powder which contains chromium andchromium carbide in each particle.

A further object is to provide a method for making such powder.

Yet another object is to provide a chromium/chromium carbide coatinghaving superior property to hard chromium electroplate.

Still another object is to provide a coated trochoid surface for arotary combustion engine.

These and other objects will either be pointed out or become apparentfrom the following description and drawings wherein:

FIG. 1 is a pictorial representation of the structure obtained bydepositing mechanical mixture of chromium and chromium carbides;

FIG. 2 is a pictorial representation of the type structure obtained bydepositing the powder of this invention;

FIGS. 3, 4 and 5 show possible distribution of the carbide phases in thepowder particles;

FIG. 6 shows the variation of wear scar volumes with carbon content ofthe powder used to produce the coating tested, compared to coatings ofhard chrome plate; and,

FIG. 7 shows the hardness of coatings obtained with powders of variouscarbon content compared to hardness of hard chrome plate.

The methods of this invention, which will be described shortly, producea composite powder containing the desired amount of chromium carbide andchromium in which substantially each particle contains at least somechromium and chromium carbide. Examples of the possible distributions ofthe carbide phases in the powder particles are shown in FIGS. 3, 4 and5. For use in producing plasma or detonation-gun coatings, the exactcomposition of the carbide phases in the powder or the distribution ofthe carbide phases as shown in FIGS. 3, 4 and 5 are not important, onlythe total carbon content, since during deposition the particles becomeessentially completely molten. As the individual splats solidify duringdeposition, the carbides reprecipitate from the melt forming Cr C Cr Cor Cr C or a combination of these, depending on the total amount of Cpresent and the rate of solidification. The preferred compositionresults in a predominantly 'Cr C dispersion.

Basically, the material is prepared by chemical reaction of an intimatemixture of a source of Cr and a source of C; temperatures of 1000-1400"C. are suitable for solid state reactions. Times of from about l-5Ohours are suitable. Temperatures in excess of l500 C. are required forproduction of the powder by melting referred to hereinafter. Theprincipal reaction involved is The principal product is Cr C with minoramount of CI'qCg and CI'3C2.

When oxygen is present in the Cr (as Cr O or Cr O is used as the Crsource, reaction (1) is preceded or accompanied by The Cr formed inreaction (2) may react with C present in excess of the amount requiredto bring reaction (2) to completion to form Cr carbide by reaction (1).

The source of Cr may be commercial Cr powder (e.g., Union Carbide Miningand Metals Division electrolytic chromium powder), Cr 'O as in reaction(2), or any compound that decomposes on heating or by reaction with C orH on heating to form essentially Cr and volatile products.

The source of carbon may be any commercial carbon consisting ofessentially elemental C and volatile impurities. Decolorizing carbon,lampblack, and powdered graphite have been used with equal success. Inaddition, a higher carbide of Cr may be used as the C source, since itmay react with Cr to form another carbide, the resulting product havingthe characteristic intimacy of the invention. As an example,

would produce a carbide on the surface of the Cr particles (present inexcess of the amount consumed in reaction (3)).

A gaseous hydrocarbon or hydrocarbon/hydrogen gas mixture is also asuitable carbon source, provided its composition is such that the carbonactivity is high enough to permit carbide formation. This reaction hasnot been used directly, but powdered mixtures of Cr and C heated in a Hatmosphere are found to consist, after reaction, of two-phase particlesin which the carbide phase essentially encapsulates the original Crparticles as shown in FIG. 3. This structure differs from that found insimilar mixtures heated in the absence of H which show mainly isolatedareas of carbide formation on the Cr particles, as shown in FIG. 4,corresponding to points of 4 solid-solid contact of the original Cr andC particles. The difference in structure is clear evidence that carbonhas been transported through the vapor phase in the H atmospbere, by thereaction occurring at the carbon particles and the reaction also occurs.

The intimately mixed Cr/ Cr carbide structure may also be prepared bymelting Cr and C (present either as the element or as a Cr carbide)mixtures of appropriate total analysis, allowing the homogeneous liquidto freeze and the Cr carbide to precipitate out, and then crushing thesolidified melt to powder. Temperatures greater than 1500 C. arerequired for this method. Limitations of higher melting temperatures anddifficulty in crushing the solidified melt practically limit this methodof preparation to carbon content of 3% by weight or more.

The reaction of Cr and C is preferably carried out in vacuum becausethis promotes the removal of the gaseous CO formed in reaction (2) or(6). The vacuum does not have to'be extraordinarily good, ultimatesystem pressures between 0.01 and microns having been found 0 to yieldproducts of essentially the same oxygen content.

The reaction can also 'be carried out in any atmosphere with oxygenpotential sutficiently low to prevent oxidation of Cr. A hydrogenatmosphere is quite suitable and is particularly useful for thepreparation of a composite of low C content with a uniform carbidedistribution, since the H takes part in the reaction and promotesuniform distribution.

The product of the Cr+C or Cr O +C reaction is a sintered cake, howeverthe reaction is carried out. Sintering is least, and reduction to powderby ball-milling, hammer-milling, and other conventional techniques iseasier, when the Cr O +C reaction is used or when the Cr+C reaction iscarried out in H Lower reaction temperatures favor ease of reductionwhen the Cr+C reaction is carried out in vacuum.

The carbide distribution within the powder particle is a. function ofthe method of production. When a mixture of solid carbon and chromium isheated in vacuum, the predominant form is that shown in FIG. 4 becausethe carbon tends to react with the chromium surface closest to it. Thefiner and more uniform the distribution of carbon in the startingmixture, the more uni-form the distribution of carbides around thesurface of the chromium will be. The ultimate extension of this trend isachieved when a gaseous source of carbon is used either by directlysupplying a hydrocarbon gas or by heating the solid carbon plus chromiumin a hydrogen atmosphere (which results in a hydrocarbon gas). Thecarbide distribution which results is like that in FIG. 3. Adistribution of carbon particles throughout the powder particle, FIG. 5,may result when a solid ingot of the proper total composition is reducedto powder.

Oxygen content (in the range 0.03 to 1%) does not affect the wearproperties of coatings made from powders of this invention. The carboncontent of the powder of this invention may be between 0.2% and 5.4% byWeight. At the lower limit, plasma deposits made from the powder aresuperior in tests to similar deposits made from commercial electrolyticchromium powder. The high end of the range is defined by the completeconversion to the compound Cr C which contains 5.6% by weight; at thispoint, the material no longer contains free Cr. The wear resistance ofcoatings made from the powder varies with carbon content as shown in theband curve on FIG. 6. The range of values observed for commercial hardchrome plate is also shown in the FIG. 6 by the cross-hatched areaadjacent to the vertical axis.

The optimum composition is believed to lie in the range 0.81.7% C. byweight, and may vary somewhat with the method of preparation. Coatings,made from powders in this composition range, are equivalent to orsuperior to commercial electrolytic Cr plate in laboratory lubricatedrubbing wear tests at high load (see FIG. -6). Furthermore, thehardness, see FIG. 7, is at a minimum, making it possible to readilyfinish the coating with conventional grinding or honing tools.Low-surface-speed, high-deposition-rate plasma plating produceswell-bonded, uncracked coatings.

Specifically, it has been found that powders containing about 1 wt.percent carbon produce plasma deposited coatings on interior trochoidsurfaces of rotary combustion engines which have remarkedly andunexpectedly superior properties, as shown hereinafter in Example 9.

The coating of this invention is characterized -by the presence insubstantially every splat of both Cr and Cr carbide. As pictoriallyillustrated in FIG. 2, the relative amounts of Cr and Cr Carbides willvary between splats as a necessary result of the use of powder with arange of partial sizes and advantitious difference in the degree towhich each Cr particle is carburized and in the conditions to which thevarious particles are subjected in passing through the coating device.Nevertheless, the coating of this invention is distinguished from thatproduced from a powder which is a simple mixture of Cr and Cr carbide,which is pictorially represented in FIG. l, in that the splats in thelatter type of coating are each individually either all Cr or all Crcarbide.

FIG. 2 is to be understood as being merely illustrative of one featureof the distribution of the carbides in the coating. Upon extraction bychemical methods of carbides from the invention and examination of thesecarbides by optical and electron microscopy, it has been found that atleast some, and probably most, of the carbides are much finer thansuggested by FIG. 2. The majority of the carbide particles were found tobe of sub-micron size and most were predominantly in the shape of alace-like network, suggesting that the coatings contained fine-grainedinterlocking, continuous networks of both carbide and Cr, the separationbetween the interstices of these networks being so small that they arenot resolvable in optical microscopy.

The coatings produced with the powder of this invention have a number ofadvantages in addition to the general procesing advantages previouslydescribed as being associated with metal spray deposition.

(1) Coatings are superior to those formed by the plasma deposition ofcommercial electrolytic chromium powder in that increased wearresistance and resistance to spalling are found, though there is minimalincrease in hardness as measured by diamond pyramid indentations.

(2) Coatings are superior to coatings in which nitrogen rather thancarbon is the strengthening additive, in that carbide-strengthenedmaterial is much less brittle and much less prone to spalling.

(3) In the laboratory lubricated rubbing wear test described in ExampleI, the coatings of this invention with a carbon content in the preferredrange of 0.8-1.7 wt. percent C, performed as well as or better thancommercial electrolytic chrome plate.

(4) Coatings of this invention performed far superior to electrolyticchrome plate coating on internal troc-hoid surfaces in rotary combustionengines as described in detail in Example 9.

The following examples illustrate the invention but are not intended tolimit the variations in processing that would be apparent to thoseskilled in the art. Moreover, the use of the powder of this invention isnot intended to be limited to plasma or detonation-gun deposition.

EXAMPLE 1 8879 grams of Union Carbide Mining and Metals Divisionelectrolytic chromium, screened through a 230-mesh sieve, was mixed with200 grams of Fisher Scientific Company Norit A decolorizing carbon,similarly screened, and blended for two hours in a cone blender. Aportion of this mixture was used to fill eight pans, each about 0.6 cm.deep, so that each pan contained between 210 and 230 grams of themixture. The pans were vertically stacked in a vacuum furnace so thatthere was about 0.4 clearance between pans. The furnace was evacuatedslowly to about 500-micron pressure and then more rapidly to about 0.5micron, using an oil-diffusion pump. Power was then applied to tantalumstrip heaters surrounding the stack of pans and the pans heated over aperiod of about minutes to a temperature of 1080" C. as indicated by athermocouple in contact with the powder in the uppermost pan; systempressure was maintained below 50 microns during this period by adjustingthe rate of heating. The powder was maintained at 1080" C. for fourhours, during which time the pressure gradually dropped to about 0.3micron. The furnace was then allowed to cool to room temperature withpumping continued. When the pans were removed from the furnace, thematerial was in the form of sintered cakes of a significantly moremetallic appearance than the original powder mix. These cakes werecrushed in a mechanical pulverizer until about of the material wasreduced to powder that would pass a 325-mesh screen. The balance of theoriginal mixture of chromium and carbon powders was processedidentically in four additional furnacings.

The 325- mesh powders from the five furnace runs were individuallyanalyzed for combined carbon, free carbon, and oxygen. All showed lessthan 0.1% free carbon, between 300 and 420 p.p.m. oxygen, and 1.05-1.08%combined carbon. The distribution of carbides on the chromium wassimilar to that in FIG. 4.

The products of the five runs were blended together and used to producecoatings by deposition through a plasma torch. Coatings so produced,when separated from the substrates on which they were plated, analyzed1.03- 1.06% C. The Wear resistance of these plasma-deposited coatingswere measured using a Dow-Corning LFW-l Friction and Wear Test Machineaccording to ASTM Standard Method D2714-68. Coatings deposited 12 milsthick on the wear surface of mild steel wear blocks were ground to afinal thickness of 5 mils and tested against carburized AISI 4620 steelrings (surface hardness 58-63 Rockwell C) at 450 lb. specific load for5400 n'ng revolutions at about rpm; MIL-5606A hydraulic fluid was usedas lubricant. Wear scar volumes, calculated from the projected scarareas and the known ring diameter, ranged between 24 and 49x10 cm. Thesetest results are included in FIG. 6.

EXAMPLE 2 Numerous mixtures differing only in the amounts ofelectrolytic chromium and decolorizing carbon used were processed asdescribed in Example 1. The resulting powders, which ranged in carboncontent from 0.6 to 5.4%, were used to form plasma-deposited coatingsand tested for wear resistance using the techniques and proceduresdescribed in Example 1. Results of these tests are included in FIG. 6.

EXAMPLE 3 5400 grams of the same electrolytic chromium powder used inpreceding examples was mixed with 87 grams of lampblack for one hour ina ceramic ball mill and then further mixed for 30 minutes in a coneblender. The mixed powders were loaded into pans and heated in thevacuum furnace exactly as described in Example 1. The product, afterreduction to 325 mesh powder, analyzed 0.81% carbon and 335 ppm. oxygen.Plasma-deposited coatings were made and tested as described inExample 1. Scar volumes of 21 to 34x10 cm. were observed; these resultsare included in FIG. 6.

EXAMPLE 4 1476 grams of the same electrolytic chromium powder used inprevious examples and 24 grams of the same screened decolorizing carbonused in previous examples were blended for two hours in a cone blender.Two boats, each 0.6 cm. deep and about 25 cm. long, were filled withthis powder and placed in a cm. diameter ceramic tube furnace which wasthen sealed and evacuated with a mechanical pump for several hours. Thefurnace was then filled wtih hydrogen, heated to 1150 C., and maintainedat this temperature for 22 hours, a flow of s.c.f.h. of hydrogen beingmaintained during the entire cycle. The

product was a sintered cake much more readily reduced to -325 meshpowder than the products of the vacuum processing previously described.This powder analyzed 1.06% carbon, 630 ppm. oxygen. Plasma-depositedcoatings made and tested as described earlier had scar volumes of 21 to42 10- cm. A portion of the powder was mounted and polished tometallographic examination; at 500x magnification, it appeared thatmost, and possibly all, of the powder particles consisted of a shell ofchromium carbide surrounding a core of chromium metal similar to that inFIG. 3. In this respect, the structure differed from that of powdersprepared by vacuum processing; in the latter carbide and metal wereobserved in the same particles, but complete encapsulation was notobserved.

EXAMPLE 5 1773 grams of the same electrolytic chromium powder used inprevious examples and 27 grams of the same screened decolorizing carbonused in earlier examples were blended by shaking and rolling in a 32-oz.glass jar. Using this powder, eight separate heats, each with between 80and 105 grams of mix, were made in a 4 cm. diameter tube furnace. Eachheat was for five hours at 1140 C. in a fiow of about 110 s.c.f.h.hydrogen without preliminary evacuation. The eight cakes were easilypowdered by light hammering and when blended together and screenedyielded a 325 mesh powder containing 1.13% C. and 1730 p.p.m. oxygen.The microstructure of this powder was very similar to that of the powderdescribed in Example 4, consisting of chromium carbide surroundingchromium; in addition, a small amount of very fine precipitates wasnoted decorating the carbide-chromium interface.

EXAMPLE 6 A powder analyzing 1.13% C prepared by the method described inExample 1 was plated onto test blocks using a detonation gun.Microstructural differences between these coatings and those formed byplasma deposition were observed consistent with the difference in methodof coating formation. For wear-test conditions identical with thoseemployed for the plasma-deposited materials, scar volumes of 15-19 10"cm. were measured on the detonation-gun coatings.

EXAMPLE 7 Four hundred lb. of Cr O was blended with 94.8 lb. oflampblack in a twin-shell blended and then more thoroughly blended in avibratory ball mill. This product was then mixed with 9.5 lb. cornstarchbinder and enough water to make a mix suitable for forming briquettes ina standard briquetting press. It was then pressed into briquettes ofabout 2-inch maximum dimension and dried to remove excess water. Thebriquetted mix, charged to a large vacuum furnace in an 19-inch-deep bedcovered with graphite plates, was heated to 1000 C. without letting thepressure exceed 5000 microns, held one hour at 1000 C. after thepressure had dropped below 2000 microns, then heated to 1400 C. and heldat that temperature for 50 hours, at the end of which time the pressurehad dropped to less than 150 microns. A portion of this product waspulverized to -325 mesh size and found to contain 1.14% C and 460 ppm.oxygen. The carbide dispersion in the powder was similar to FIG. 4. Wearsamples formed from this material by plasma deposition and tested asdescribed previously exhibited wear scars of 21-24 10 cm. volume.

EXAMPLE 8 A mixture of 9900 grams of commercial grade electrolyticchromium sized to pass through a 65-mesh screen and grams of lampblackwas blended dry, then mixed with water and cornstarch binder and formedinto briquettes as described in Example 7. The briquetted mixture wasthen furnaced in vacuum under graphite covers for one hour to 1000 C.and for eight hours at 1385 C. The pressure in the furnace wasmaintained below 500 microns and was 50 microns at the end of theheating period. This material was then crushed, yielding about 30% 325mesh material that analyzed 1.3% C and 721 ppm. oxygen, with a carbidedispersion similar to FIG. 4. Wear samples made from this powder byplasma deposition and tested in the standard manner exhibited wear scarsof 18-23 x 10* cm.

EXAMPLE 9 Plasma deposited coatings produced in a manner similar toExample 1 were applied to the interior trochoid surfaces of rotarycombustion engines fitted with graphite-aluminum composite rotor apexseals. The engines were run in laboratory test stands and in testvehicles. The trochoids were made of several different types ofmaterials and of two different sizes, examples of which are shown inTable I. Over 3113 hr. of test stand operation have accumulated on thesmall engine size and 331 hr. of test stand and 7000 hr. of vehicleoperation on the large engine size. In comparison with hardelectroplated chromium the coatings of this invention showed thefollowing advantages:

(a) Essentially no wear of the coated surface has been observed and noroughening or wash boarding, as occurs with electroplated chromium, hasdeveloped.

(b) The wear of the mating seal surface is approximately one-half thatcaused by hard electroplated chromium, which is greater than .005 per100 hr.

(c) Performance of the coating is less sensitive to surface finish thanhard electroplated chromium. There was no appreciable dilference in wearof either the coated surface or the seal surface between as-groundcoating sur faces of 16 to 32 microinches rms and honed surfaces ofapproximately 6 rms. In comparison, a hard electroplated chromiumsurface must be finished to better than 6 microinches rms to performsatisfactorily.

(d) Finishing of the plasma-deposited coating is far simpler and may becheaper since it can be used as-ground while a hard electroplatedchromium coating must be ground, then etched to enhance the microcracked texture of the surface, and then honed to improve the surfacefinish. Because thickness control is better with plasma deposition thanwith electroplating, the amount of material that must be removed infinishing is also less.

(e) The performance of engines with the coatings of this invention isfar less sensitive to fluctuation in coolant temperatures than thosecoated with hard electroplated chromium.

(f) The performance of engines with the coatings of this invention isfar less sensitive to fluctuation in oil lubrication than that ofengines coated with hard electroplated chromium. The latter requirecontinuous addition of oil to the combustion chamber, but an engineprovided with the coating of this invention continued to performsatisfactorily when the oil addition was stopped.

TABLE I Total Average Coating time seal wear Trochoid thickness of terate (1n.l

Troehoid type size (in.) (1n.) (hr.) 100 hr.)

luminum with- 1 Major axis x minor axis x width.

Having described the invention with reference to certain preferredembodiments, it should be understood that minor modifications can bemade thereto without departing from the spirit and scope thereof.

What is claimed is:

1. A powder containing from about 0.2 wt. percent to about 5.4 wt.percent carbon and wherein substantially every particle of said powderconsists essentially of chromium and at least one chromium carbide takenfrom the class consisting of Cr C Cr C and Cr C 2. A powder according toClaim 1 wherein said powder contains from about 0.8-1.7 wt. percentcarbon.

3. A powder according to Claim 1 wherein said powder contains about 1wt. percent carbon.

4. A powder according to Claim 1 wherein each particle has a core ofchromium substantially completely surrounded by a shell of said chromiumcarbides.

5. A powder according to Claim 1 wherein each particle contains chromiumand said chromium carbides on the surface of said chromium.

6. A powder according to Claim 1 wherein each particle contains chromiumand said chromium carbides dispersed within said chromium.

7. An article consisting of a metal substrate having a coating thereonconsisting essentially of a composite of chromium and at least onechromium carbide taken from the class consisting of Cr C CI'7C3 and Cr Cwherein the composite coating contains from 0.2 wt. percent to about 5.4wt. percent Carbon and the composite is characterized by a multilayerstructure of overlapping thin,

10 lenticular particles, each particle containing a mixture of saidchromium and chromium carbides.

8. Article according to Claim 7 wherein the carbon content is in therange of 0.8-1.7 wt. percent.

9. A coated trochoid surface of a rotary combustion engine comprising ametallic surface having a coating thereon consisting essentially of acomposite of chromium and at least one chromium carbide taken from theclass consisting of Cr C Cr C and Cr C wherein the composite coatingcontains from 0.2 wt. percent to about 5.4 wt. percent Carbon and thecomposite is characterized by a multilayer structure of over-lappingthin, lenticular particles, each particle containing a mixture of saidchromium and chromium carbides.

10. A coated trochoid surface of a rotary combustion engine according toClaim 9 wherein the carbon content is in the range of 0.81.7 wt.percent.

11. A coated trochoid surface of a rotary combustion engine according toClaim 9 wherein the carbon content is about 1 wt. percent.

12. An article consisting of a composite of chromium and at least onechromium carbide taken from the class consisting of Cr C Cr C and Cr Cwherein the composite contains from 0.2 wt. percent to about 5.4 wt.percent carbon and the composite is characterized by a multilayerstructure of over-lapping thin, lenticular particles, each particlecontaining a mixture of said chromium and chromium carbides.

References Cited UNITED STATES PATENTS 2,398,132 11/1941 Cottrell -0.513C 3,254,970 6/1966 Dittrich et a1. 29-1912 X 3,258,817 7/1966 Smiley29-1912 X 3,419,415 12/1968 Dittrich 117-93.1 DF 3,488,291 1/ 1970 Hardyet a1. 750.5 AC 3,655,425 4/1972 Longo et al. 106-1 3,723,601 3/ 1973Svanstrom 423-440 X 3,732,091 5/ 1973 Paris et al 750.5 AC 3,752,6558/1973 Ramquist 750.5 BC

FOREIGN PATENTS 228,798 5/ 1960 Australia 117-1052 WINSTON A. DOUGLAS,Primary Examiner O. F. CRUTCHFIELD, Assistant Examiner US. Cl. X.R.

117-9 3.1 PF, 105.2; 106-1; 75-0.5 AC, 0.5 BC

1. A POWDER CONTAINING FROM ABOUT 0.2 WT. PERCENT TO ABOUT 5.4 WT.PERCENT CARBON AND WHEREIN SUBSTANTIALLY EVERY PARTICLE OF SAID POWDERCONSISTS ESSENTIALLY OF CHROMIUM AND AT LEAST ONE CHROMIUM CARBIDE TAKENFROM THE CLASS CONSISTING OF CR23C6;CR7C3 AND CR3C2.